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In situ denitrification on nitrate rich groundwater in South Africa

Israel, Sumaya (2015-12)

Thesis (PhD)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: South Africa is a water scarce country and in certain regions the quantity of surface water is insufficient to provide communities with their domestic water needs. In many arid areas groundwater is often the sole source of water. This total dependence means that groundwater quality is of paramount importance. A high nitrate concentration in groundwater is a common cause of water being declared unfit for use and denitrification has been proposed as a potential remedy. In many areas of South Africa nitrate levels exceed the recommended maximum concentration of 40 mg/L NO3- as N. Concentrations of 100 mg/L NO3- as N or even greater than200 mg/L NO3- as N are found in various places. Water with nitrate concentrationsexceeding 40 mg/L NO3- as N, belongs to the category of “dangerous” drinking waterquality (“purple”, i.e. Class IV) according to DWA (1996, 1998) water quality guidelines. Concentrations in this range have been reported in case studies to cause conditions like methaemoglobinaemia (“blue baby syndrome”), spontaneous abortions, stomach cancers and livestock deaths.
The purpose of the study includes laboratory experiments to compare the denitrification efficiency, reaction rates and reaction mechanisms between woodchips, biochar and a mixture of woodchips and biochar. Further work included modelling of denitrification using the PHREEQC-2 1D reactive transport model. Field implementation of a denitrification technique was tested at a site which previously experienced some NH4NO3 spills, to determine the lifespan of the woodchips used during the experiment based on available data. The underlying intended purpose of this research is to contribute to the wellbeing of rural South Africans in areas where groundwater is plentiful, but elevated nitrate levels prevent the use of this water.
The purpose of the laboratory experiment was to establish the efficiency of carbon sources and compare their rates, sorption properties and processes by which they react. Laboratory experiment consisted of three leaching columns containing two layers of building sand on either side of a carbon containing layer. The carbon containing layers were made of about 600g of woodchips, biochar and woodchip and biochar mixture respectively. Parameters analysed from the effluent from the columns included NO3-, NO2-, SO42-, NH4+, Alkalinity, DOC (dissolved organic carbon content) and Phosphate.
The purpose of the field experimental work was to install a barrier containing a cheaply available carbon source to treat groundwater and to monitor changes with time in order to determine the efficiency and life span of carbon source used for the experiment. Experimental work was done at a site in Somerset West (South Africa) that had experienced spills in the past from agrochemical storage factory premises. Somerset West normally receives about 568 mm of rain per year. It receives most of its rainfall during winter; it thus has a Mediterranean climate. It receives the lowest rainfall (10 mm) in February and the highest (96 mm) in June. The “reactor”/ tank with dimensions- 1,37m height, 2.15m diameter used for the experiment was slotted for its entire circumference by marking and grinding through the 5mm thick plastic material. The top section was left open to allow for filling and occasional checking of filled material during the experiment. The tank was packed with Eucalyptus globulus woodchips which was freely available at the site.
Concentrations of groundwater nitrate at the site were well over what could be expected in any naturally occurring groundwater systems, and would result only by major anthropogenic activities in unconfined aquifer areas of South Africa. Nitrate levels in monitoring boreholes at the site ranged from about 20 mg NO3--N/L at background boreholes up to about 600 mg/L NO3--N. Woodchips used to denitrify groundwater in the field experiment were sampled after 27 months and 35 months of being active in the treatment zone. Various depths of samples were collected namely the top section, bottom of the tank and a full core sample of the tank.
Main results from the laboratory studies showed that biochar on its own as a carbon source for nitrate removal would not be viable, however, the presence of biochar in the mixture of woodchips and biochar increased the rate of denitrification. Biochar on its own was able to remove some nitrate, but results showed incomplete denitrification and limited reactivity. The results also confirmed that different processes were in play, while the redox reaction of denitrification was taking place in woodchips and biochar and woodchip mixtures, the biochar treatment followed a physical process and had only a small percentage of incomplete denitrification. This was confirmed by sulphate reduction and increased alkalinity in the woodchips and biochar and woodchip mixture treatments. Rates deduced from the data also showed that the woodchip and biochar mixture would take a shorter period to affect total denitrification.
Main results from the field work showed that nitrate was totally removed at the treatment zone and surrounding boreholes, and even sulphate and NH4+ were removed during the experiment. This shows that the woodchips were successful in affecting denitrification for 35 months. Data also shows that boreholes further downstream from the tank had reduced NO3-, SO42- and NH4+ levels. This would relate to higher permeability flow paths possibly present on the downstream side of the treatment zone. This became evident when pumping boreholes during sampling and noting that upstream boreholes had to be allowed for a recovery period, while downstream boreholes could be pumped continuously for 30 minutes without any reduced yields. This shows that not only did the treatment zone work at removing nitrate, but migration of excess available carbon from the tank may have further treated nitrate rich areas on the site.
During monitoring on the site, woodchips were sampled and analysed for their components at time period 27 months and 35 months of the experiment respectively. Results showed that woodchips were considerably more degraded than a) woodchips of the same species of tree that had undergone natural degradation on the floor and b) un-degraded woodchips of the same tree species. Comparing data from the two time series samples, a rate of woodchip degradation could be calculated. Using the available biodegradable carbon for the woodchips based on its composition, a barrier lifespan could be determined. The results of calculations show that the barrier would be effective for at least another 6.9 years from the period of the last sampling date. A total lifespan of about 10 years can thus be estimated. These calculations are tree species composition specific and rate specific.
PHREEQC-2 modelling was used to estimate the use of carbon in the experiment by adding incremental moles of carbon to the influent composition. Saturation indices from PHREEQC-2 showed that mineral phases of iron may precipitate from solution during the experiment. Experimental data were plotted against results of intermittent carbon reactions in PHREEQC-2 and it was found that initial rates in the experiment were higher and agreed with up to 100mg/L of carbon consumption when a 24 hour residence time was used while later stages agreed with about 37.5 mg/L carbon consumption, where a 72 hr residence time was used.
It was concluded that biochar and woodchips combined are more effective than woodchips on their own at denitrifying groundwater. Also woodchips successfully denitrified groundwater at the Somerset West site for 35 months, with added removal of sulphate and NH4+. Barrier life span calculations show that the barrier could remain active for an additional 6.9 years which relates to a total period of about 10 years of denitrification should the rates remain constant. It was concluded that nitrate removal and barrier lifespan would be extended by testing variable lignin content in different tree species prior to use in a denitrification barrier as lignin is unlikely to degrade in an anaerobic environment.
It was recommended that implementation or field test should be done using a biochar and woodchip mixture. Improved results may be achieved by analysing wood or plant material for comparative lignin content, cellulose content and hemicellulose contents. Wood types or plant species with higher lignin content would be more resistant to degradation in anaerobic conditions.